Many different measurement techniques have been used previously as the basis for displacement sensors.
In one type of displacement sensor the action of linearly or rotationally displacing a wiper of a potentiometer is converted to a voltage and/or current signal. Such potentiometric sensors often suffer from the problems of mechanical wear, frictional resistance in the wiper action, limited resolution in wire-wound units, and high electronic noise.
Linear Variable Displacement Transducers (LVDT) are commonly available. An LVDT typically includes three coils of wire wound on a hollow form. A core of permeable material can slide freely through the centre of the form. The inner, primary coil is excited by an ac source. Flux formed by the primary coil is linked to two outer, secondary coils, inducing an ac voltage in each coil depending on the position of the core. If the two secondary coils are wired in series opposition then the two voltages will subtract; that is, a differential voltage is formed. When the core is centrally located, the net voltage is zero. When the core is moved to one side, the net voltage amplitude will increase. In addition, there is a change in phase with respect to the source when the core is moved to one side or the other. Additionally, these devices require separate coils at either end of the measurement coils to provide electrical shielding to create a low noise transducer. These manufacturing requirements make these transducers expensive to manufacture and have a length dimension at least twice the distance they can measure.
A number of devices have also been described based on optical measurement systems such as optical encoders. Devices based on ultrasonic techniques have also been described. These devices tend to be expensive to manufacture and are restricted in the type of application in which they can be employed.
A variety of capacitance based displacement sensors have been described for measuring or detecting linear displacements. One type of capacitance displacement sensor is based on the principle of two opposing plates, where measurement of displacement either alters the overlapping area of the two plates or changes the dielectric properties of the gap between the plates. Examples of this type of displacement sensor are offered below.
GB 1275060 A discloses a displacement sensor comprising of guided rod forming a first plate of the capacitor and a receptor tube in which the rod moves in and out forming a second pate of the capacitor.
U.S. Pat. No. 4,961,055 discloses a displacement sensor similar to that of GB 1275060 A and further discloses a third tube that acts to shield the sensing plate of the capacitor from electrostatic charges, which can cause signal noise.
A number of other moving plate capacitor sensors have also been described that utilise patterns of electrodes on either flat or tubular plates. Examples of these are given by JP 8-159704 and GB 2273567 A. The construction of these devices also presents considerable challenges in manufacturing inexpensive devices.
The use of capacitance displacement sensors has been described for a variety of applications including monitoring fluid levels in reservoirs, as disclosed in EP 0520201 A.
U.S. Pat. No. 5,135,485 discloses a capacitance measurement employed in a drug reservoir to either detect when the reservoir is empty or provide a measure of the level of liquid in the reservoir. The sensor described for monitoring the level of liquid in the reservoir comprises two plates of a capacitor with the liquid forming the dielectric between them. The greater the quantity of liquid present in the reservoir the more the gap between the plates becomes filled with the liquid and this is reflected in the capacitance measured by the sensor.
U.S. Pat. No. 6,210,368 discloses a capacitor based sensor that monitors liquid levels in a reservoir. In one embodiment an amount of overlap between two plates of a capacitor changes as the reservoir volume changes. In another embodiment an amount of liquid phase propellant absorbed in a dielectric material of a capacitor changes according to the reservoir volume, causing a change in the dielectric properties of the capacitor.
U.S. Pat. No. 6,352,523 discloses a method for measuring the amount of insulin remaining in a syringe after an administration based on a barrel and plunger of a syringe being adapted as the two plates of a coaxial capacitor. The device additionally requires that the syringe is placed into a reader to generate the displacement information.
Alternative techniques for monitoring levels of a drug in a reservoir include the use of optical encoders. U.S. Pat. No. 4,498,843 and WO2004/009163 both describe a linear displacement measurement system based on an optical encoder that is used to monitor the position of a syringe barrel as part of an infusion system.
There is a need in the art for a displacement sensor capable of monitoring the level of liquid in a syringe type drug reservoir with sufficient sensitivity as to allow detection of erroneous drug delivery. It is also required that the sensor is inexpensive to manufacture and provides reliable performance through robust design.